Stainless steel powder for producing duplex sintered stainless steel

ABSTRACT

Embodiments of the present invention may provide a new stainless steel powder suitable for manufacturing of duplex sintered stainless steels. Embodiments of the present invention may also relate to a method for producing the stainless steel powder, the duplex sintered stainless steel as well as methods for producing the duplex sintered stainless steel.

TECHNICAL FIELD

Embodiments of the present invention may provide a new stainless steelpowder suitable for manufacturing of duplex sintered stainless steels.Embodiments of the present invention may also relate to a method forproducing the stainless steel powder, the duplex sintered stainlesssteel as well as methods for producing the duplex sintered stainlesssteel.

BACKGROUND

Duplex stainless steels have been known to the industry for more than 60years. They are widely used in heat-treated cast, wrought and gasatomized powder forms, in many applications that require a combinationof high strength and high corrosion resistance. However, they areunavailable today, in the water atomized powder form for use in pressand sinter applications.

Common uses for duplex stainless steels include chemical process plantspipeline, petrochemical industry, power plants and automobiles. They arealso used in food processing industry, pharmaceutical processcomponents, paper and pulp industry, in desalination plants and in themining industry. Duplex stainless steels are known for their highresistance to inter granular corrosion (IGC) and stress corrosioncracking (SCC) in chloride media. Chloride is severe challenge thatleads to rapid corrosion media for iron-based alloys.

High strength and high corrosion resisting properties in duplexstainless steel are believed to be acquired due to a presence of ferriteand austenite phases in equal amounts. Such structure is generallyachieved by using a balance of austenite stabilizers, e.g., nickel (Ni),manganese (Mn), carbon (C), nitrogen (N), copper (Cu) and cobalt (Co),and ferrite stabilizers, e.g., chromium (Cr), silicon (Si), molybdenum(Mo), tungsten (W), titanium (Ti) and niobium (Nb).

As mentioned previously, the high strength and high corrosion resistanceof duplex stainless steel is believed to come from a balance of ferriteand austenite in the microstructure. The microstructure depends not onlyon the chemistry but also on the heat treatment carried out on thematerial. All duplex steel compositions today make use of N in thechemistry, as N is a strong austenite stabilizer. N, when present in thealloy along with Cr, poses problem of forming nitrides which aredeleterious to the properties such as strength and corrosion resistance.Further, during welding duplex stainless steels, an intermetallic phaseknown as “Sigma” is formed in a heat affected zone (HAZ) due to slowercooling rates. This Sigma phase is a hard, supersaturated, intermetallicphase containing Cr and Mo. The area around the Sigma phase is depletedof Cr and Mo and becomes weak and less resistant to corrosion. Oftenduplex stainless steels need annealing and quenching process to reduceor eliminate this Sigma phase.

In wrought or cast duplex stainless steels, the steel is solidified asferritic steel and the austenite phase is precipitated out from ferriteduring cooling of the alloy. The cooling rate is critical after castingor at any heat treatment, as the cooling rate determines the percentageof austenite and any intermetallic phases, precipitated within thestructure.

Although wrought duplex stainless steels, in particular ‘hot rolled’duplex stainless steels, have been common in industrial use since 1930s,they were hardly used in the Powder Metallurgy (PM) industry. There area few applications where gas atomized duplex stainless steel powders areused in hot isostatic pressed (HIP) condition. Powders produced by gasatomizing have spherical morphology. Such powders are less suitable forconventional press and sinter applications. Due to the spherical shape,they have insufficient green strength, which is required to handle greenpress and sinter parts. Irregular shaped powders, such as those producedwith water atomization, have much higher green strength as the irregularshape of the powders tends to bind together the powder particles.Currently there is no water atomized stainless steel powder availablefor producing sintered duplex stainless steel components. The currentchemical compositions used in gas atomized powders, and also in wroughtsteels, use N as a major alloying element to achieve austenite-ferritebalance and achieve required mechanical strength. Inclusion of N in thepowder increases the hardness of the powder reducing the compressibilityin conventional press and sinter applications. This may result inreduced green density and subsequently reduced sinter density.

There have been several attempts to develop sintered duplex stainlesssteels made from water atomized powders. Lawley et al¹ attempted todevelop equivalent grades of AISI 329 and AISI 2205 with maximum tensilestrength of 578 MPa. Dobrzanski et al² mixed ferritic and austeniticpowders to produce duplex structure with tensile strength 650 MPa. Thesame group also studied the corrosion properties of duplex stainlesssteel with electrochemical method and concluded that the duplexstainless steels show better corrosion resistance than their austeniticcounterpart³. Due to their high alloy content, these steels aresensitive to the composition and also the processing parameters. Thesealloys form intermetallic phases known as sigma, chi and gamma primewhich are rich in Mo, W, N, Ni and Cr and reduce both mechanicalproperties and corrosion properties. Sigma phase forms in a temperaturerange 700° C. to 1000° C. whereas Chi phase forms within range 300° C.to 450° C. The Gamma (austenite) phase may start forming at around 600°C. ¹A. Lawley, E. Wagner, C. T. Schade, Advances in Powder Metallurgyand Particulate Materials 2005 Part 7 pp 78-89²L. A. Dobrzanski, Z.Brytan, M. Actis Grande, M. Rosso, Archives of Materials Science andEngineering, Vol 28 Iss 4, April 2007 PP 217-223³L. A. Dobrzanski, Z.Brytan, M. Actis Grande, M. Rosso, Journal of Achievements in Materialsand Manufacturing Engineering, Vol 17 Iss 1-2 pp 317-320

Typical composition of wrought duplex stainless steel is Fe with 21-23wt % Cr, 4.5-6.5 wt % Ni, 2.5-3.5 wt % Mo, and 0.08-0.2 wt % N, such asfor SAF 2205. There are numerous patents for duplex stainless steelcomposition close to this composition. Almost all of the duplexstainless steels rely on the N content for increased corrosionresistance and increased strength. So far the commercial uses ofsintered powder metallurgy (PM) duplex stainless steels are limited tothe use of gas atomized fine powders that can be used for mainly HIPprocess. The main obstacle in using low cost water atomized powders forconventional PM use is increased N and possibility of intermetallic andcarbide precipitation due to cooling rate during the sintering. Alsoconventional sintering needs some wetting agents or low temperaturemelting constituents to increase free energy and accelerate the kineticsof austenite phase precipitation within ferritic matrix.

In the patent literature there are some documents disclosing sinteredduplex stainless steel structures.

SE538577C2 (Erasteel) discloses a sintered duplex stainless steel madefrom gas atomized powder and having a chemical composition with a max0.030 wt % C, 4.5-6.5 wt % Ni, 0.21-0.29 wt % N, 3.0-3.5 wt % Mo, 21-24wt % Cr, and optionally one or more of 0-1.0 wt % Cu, 0-1.0 wt % W,0-2.0 wt % Mn, 0-1.0 wt % Si wherein N is equal or greater than 0.01*wt% Cr and the remaining elements are Fe and unavoidable impurities.

EP0167822A1 (Sumitomo) discloses a sintered stainless steel comprising amatrix phase and a dispersed phase and a process for manufacturing. Thedispersed phase is an austenite metallurgical structure and is dispersedthroughout the matrix phase, which is comprised of an austeniticmetallurgical structure having a steel composition different from thatof the dispersed phase or a ferritic-austenitic duplex stainless steel.

JP5263199A (Sumitomo) discloses production of a sintered stainless steelcomprising a matrix phase and a dispersing phase. The method includesmixing a ferritic stainless steel powder with a powder selected from anaustenitic stainless steel powder, an austenitic-ferritic duplexstainless steel powder, an austenitic-martensitic duplex stainless steelpowder and austenitic-ferritic-martensitic stainless triple phasestainless steel powder. The powder mixture being compacted and sintered.

EP0534864B1 (Sumitomo) discloses a sintered stainless steel having acontent of N of 0.10-0.35 wt % and made from gas atomized steel powderhaving the same chemical composition as the sintered stainless steel.

SUMMARY

Almost all duplex grades available have N content between 0.18-0.40 wt %in order to balance austenite-ferrite balance in the structure andincrease the strength. Although N content helps the above properties, itcan pose hurdles in post processing, such as heat treatment and weldingoperations, by forming chromium nitrides, which limits the use of duplexstainless steels in many applications. In powder form, N increases thepowder hardness making it less suitable for press and sinterapplications.

Embodiments of the invention overcome the problem with nitrides byavoiding the use of N in the chemistry, for example, having less than0.10 wt % N or less than 0.07 wt % N, or less than 0.06 wt % N, or lessthan 0.05 wt % N, or less 0.04 wt % N, or less than 0.03 wt % N, andachieving phase balance and strength by alternative elements.Embodiments of the invention may enable production of water atomizedpowder with moderate compressibility for use in conventional press andsinter applications. Embodiments of this composition may also reduceprecipitation of a deleterious ‘Sigma’ phase; irrespective of rate ofcooling during sintering or annealing, mainly due to lower Mo content.Thus, minimizing post sintering heat treatments necessary to eliminate“Sigma” phase and minimizing sigma phase precipitation during welding.

Embodiments of the composition may offer similar advantages when formedby gas atomization.

Other than conventional PM, embodiments of the composition yield similarproperties when processed with casting, direct metal deposition andadditive manufacturing techniques.

DETAILED DESCRIPTION

One object of certain embodiments of the invention is to provide analloy powder for conventional PM that will produce a duplex structureduring a sintering cycle.

Another object of certain embodiments of the present invention is toprovide a duplex sintered stainless steel.

Another object of certain embodiments of the present invention is toobtain at least 35% higher tensile strength than ferritic steels such as430L and double the corrosion resistance compared to austenitic steelssuch as 316L.

Still another object of certain embodiments of the present invention isto provide a method for producing a duplex sintered stainless steelwithout the need of post sintering heat treatment.

The above objectives may be accomplished by the following aspects andembodiments.

In a first aspect of the present invention there is provided a stainlesssteel powder comprising, or consisting of, in weight percent:

-   -   up to 0.1% of C,    -   0.5-3% of Si,    -   up to 0.5% of Mn,    -   20-27% of Cr,    -   3-8% of Ni,    -   1-6% of Mo,    -   up to 3% of W,    -   up to 0.1% N,    -   up to 4% of Cu,    -   up to 0.04% of P,    -   up to 0.04% of S,    -   unavoidable impurities up to 0.8%,    -   optionally one or more of up to 0.004% B, up to 1% Nb, up to        0.5% Hf, up to 1% Ti, up to 1% Co,    -   rest Fe.

The unavoidable impurities do not include the listed elements of C, Si,Mn, Cr, Ni, Mo, W, N, Cu, P, S, B, Nb, Hf, Ti, or Co. Unavoidableimpurities may include impurities that cannot be controlled, orcontrolled with difficulty, during manufacture of steels. These can comefrom the raw materials used and also from the process. These include,Al, O, Mg, Ca, Ta, V, Te, or Sn. The unavoidable impurities may be up to0.8%, up to 0.6%, up to 0.3%. An unavoidable impurity may be O. O may bepresent up to 0.6%, up to 0.4%, or up to 0.3%. Another unavoidableimpurity may be Sn present up to 0.2%, content of Sn above 0.2% is inthis context not regarded as an unavoidable impurity and thus will beregarded as intentionally added.

In a preferred embodiment of the first aspect there is provided astainless steel powder consisting of, in weight percent:

-   -   up to 0.06% of C,    -   1-3% of Si,    -   up to 0.3% of Mn,    -   23-27% of Cr,    -   4-7% of Ni,    -   1-3% of Mo,    -   0.8-1.5% of W,    -   up to 0.07% N,    -   1-3% of Cu,    -   up to 0.04% of P,    -   up to 0.03% of S,    -   unavoidable impurities up to 0.8%,    -   optionally one or more of up to 0.004% B, up to 1% Nb, up to        0.5% Hf, up to 1% Ti, up to 1% Co,    -   rest Fe.

In another preferred embodiment of the first aspect there is provided astainless steel powder comprising in weight percent:

-   -   up to 0.03% of C,    -   1.5-2.5% of Si,    -   up to 0.3% of Mn,    -   24-26% of Cr,    -   5-7% of Ni,    -   1-1.5% of Mo,    -   1-1.5% of W,    -   up to 0.06% N,    -   1-3% of Cu,    -   up to 0.02% of P,    -   up to 0.015% of S,    -   unavoidable impurities up to 0.8%,    -   optionally one or more of up to 0.004% B, up to 1% Nb, up to        0.5% Hf, up to 1% Ti, up to 1% Co,    -   rest Fe.

In embodiments of the first aspect the powder is ferritic. For example,99.5% ferritic. Slight amounts of austenite, e.g., up to 0.5% may betolerated.

In embodiments according to the first aspect the powder is produced bywater atomization.

In embodiments of the first aspect the powder is produced by gasatomization.

In embodiments of the first aspect the particle size of the powder isbetween 53 microns and 18 microns such that at least 80 wt % of theparticles are less than 53 microns and at most 20 wt % of the particlesare less than 18 microns.

In embodiments of the first aspect the particle size of the powder isbetween 26 microns and 5 microns such that at least 80 wt % of theparticles are less than 26 microns and at most 20 wt % of the particlesare less than 5 microns.

In embodiments of the first aspect the particle size of the powder isbetween 150 microns and 26 microns such that at least 80 wt % of theparticles are less than 150 microns and at most 20 wt % of the particlesare less than 26 microns.

In a second aspect of the present invention there is provided a methodof producing a stainless steel powder according to the first aspectcomprising the steps of:

-   -   providing a molten metal of having a chemical composition        corresponding to the chemical composition of the stainless steel        powder according to the first aspect;    -   subjecting a stream of the molten metal to water atomization;        and    -   recovery of the obtained stainless steel powder.

In a third aspect of the present invention there is provided a sinteredduplex stainless steel having a chemical composition according to thefirst aspect and embodiments thereof.

In embodiments of the third aspect the Ni equivalent (Ni_(eq)) is suchthat 5<Ni_(eq)<11 and the Cr equivalent (Cr_(eq)) is such that27<Cr_(eq)<38.

In embodiments of the third aspect the pitting resistance equivalentnumber (PREN) is 28<PREN<33.

In embodiments of the third aspect, the microstructure of the sinteredduplex stainless steel is characterized by austenite phase precipitatedwithin ferrite phase.

In embodiments of the third aspect, the microstructure of the sinteredduplex stainless steel contains 30-70% austenite and 30-70% ferrite. Inembodiments of the third aspect, the microstructure of the sinteredduplex stainless steel contains at least 99.5% austenite and ferrite,for example, at least 99.8% austenite and ferrite. The percentage ofaustenite and ferrite may be determined by ASTM E 562-11 and ASTM E1245-03.

In embodiments of the third aspect the microstructure of the sinteredduplex stainless steel is characterized by being free from sigma phasesand nitrides, for example, having less than 1% of sigma phases andnitrides.

In a fourth aspect of the present invention there is provided a methodfor producing a sintered stainless steel comprising the steps of:

-   -   providing a stainless steel powder according to the first        aspect,    -   optionally mixing the stainless steel powder with a lubricant        and optionally other additives,    -   subjecting the stainless steel powder or the mixture to a        consolidation process forming a green component,    -   subjecting the compacted green component to a sintering step in        an inert or reducing atmosphere or in vacuum at a temperature        between 1150° C. to 1450° C., preferably at a temperature        between 1275° C. to 1400° C. for a period of time of 5 minutes        to 120 minutes,    -   subjecting the sintered component to a cooling step down to        ambient temperature.

Examples of an inert atmosphere include nitrogen, argon, and vacuum withargon backfill.

An example of a reducing atmosphere is a hydrogen atmosphere, anatmosphere of a mixture of hydrogen and nitrogen, or an atmosphere ofdissociated ammonia. In limited examples, carbon dioxide or carbonmonoxide atmospheres may be used.

In embodiments of the fourth aspect said consolidation process includesthe steps of:

-   -   uniaxial compaction at a compaction pressure of up to 900 MPa in        a die to form a green component,    -   ejecting the obtained compacted green component from the die.

In embodiments of the fourth aspect said consolidation process includesone of: Metal Injection Molding (MIM), Hot Isostatic Pressing (HIP) orAdditive Manufacturing techniques such as Binder Jetting.

Methods according to the fourth aspect may include one of Laser PowderBed Fusion (L-PBF), Direct Metal Laser Sintering (DMLS) or Direct MetalDeposition (DMD).

In embodiments of the fourth aspect forced cooling or quenching isexcluded from the cooling step.

Effect of Alloying Elements

The effect of common alloying elements in stainless steels is wellknown. Cr is a major element in stainless steels which forms a Cr₂O₃layer on the surface which then prevents further oxygen passing thelayer, therefore providing an increased corrosion resistance. Ni isanother major element which affects the properties of stainless steel.Ni increases the strength and toughness of the steel and also whenpresent with Cr, enhances the corrosion resistance. Mo and W both impartthe strength and toughness when present along with Ni. Mo also enhancesthe corrosion resistance along with Cr and Ni. Si acts as deoxidizerpreventing O combining in the steel during melting, Si is also a strongferrite former. Cu is austenite stabilizer. Cu also increases thecorrosion resistance of stainless steel. Especially in conventional PM,Cu helps sintering by promoting liquid phase sintering.

Embodiments of the invention provide a powder suitable for producingsintered duplex stainless steel, as well as the sintered stainlesssteel. The powder and the sintered stainless steel having a low orneglectable content of N. This eliminates the problem of formation ofdeleterious nitrides during fabrication of the sintered stainless steel.The sintered stainless steel is preferably produced from a compacted andsintered water-atomized powder since the low N content makes it possibleto produce water-atomized powder with reasonable compressibility.

Mo is normally present in stainless steel as it strongly promotes theresistance to both uniform and localized corrosion. Mo stronglystabilizes ferritic microstructure. At the same time Mo is prone toprecipitate Mo rich “Sigma” and “Chi” phases at ferrite-austenite grainboundary. These are deleterious phases and affect strength and corrosionresistance adversely. However, due to lower Mo content in embodiments ofthe powder of the present invention, the possibility of forming sigmaphase at any cooling rate is reduced, eliminating or reducing the needfor the post processing heat treatment of annealing. This also meansthat the sigma phase will not likely form during welding operation,which is a common fabrication process for duplex stainless steels.

Cr gives stainless steels their basic corrosion resistance and increasesthe resistance against high temperature corrosion.

Ni promotes an austenitic microstructure and generally increasesductility and toughness. Ni has also a positive effect as it reduces thecorrosion rate of stainless steels.

Cu promotes an austenitic microstructure. The presence of Cu in thepowder of the present invention facilitates the sintering process byenabling liquid phase sintering.

W is expected to improve the resistance against pitting corrosion.

Si increases strength and promotes a ferritic microstructure. It alsoincreases oxidation resistance at high temperatures and in stronglyoxidizing solutions at lower temperatures.

When present in the powder according to certain embodiments of thepresent invention B, Nb, Hf, Ti, Co may enhance the properties. B whenadded in small % may help in liquid phase sintering. However, excess B,if present, may form borides, which are deleterious to both mechanical,and corrosion properties. Nb and Hf when present may stabilize themicrostructure by preferentially combining with carbon forming finecarbides freeing Cr for the corrosion resistance. Ti in stainless steelsmay increase the tensile strength and toughness. Co increases the hightemperature mechanical properties.

Elements such as C, Mn, S and P should be kept at a level as low aspossible in the powder of embodiments of the present invention as theymay have a negative effect to various extent on compressibility of thepowder and/or mechanical and corrosion preventive properties of thesintered component.

Other elements, here designated as unavoidable impurities, may betolerated up to a content of 0.8% by weight of the powder according tothe present invention.

The composition of the powder according to embodiments of the presentinvention is designed such that the produced powder will have fully(e.g., at least 99.5%) ferritic structure in the powder form andaustenitic phase is precipitated out during sintering cycle. This willallow controlling the ratio of ferrite and austenite by adjusting thesintering parameters.

Ni and Cr equivalents are calculated based on following empiricalformulae:

Cr_(eq)=Cr+2Si+1.5Mo+0.75W

Ni_(eq)=Ni+0.5Mn+0.3Cu+25N+30C

Where Cr, Ni, etc. are the level of each element in the alloy in weight%.

Further Pitting Resistance Equivalent Number is calculated as:

PREN=Cr+3.3Mo+16N

Where Cr, Mo and N are the level of each element in the alloy in weight%.

The composition is targeted such that 5<Ni_(eq)<11 and 27<Cr_(eq)<38.This places the alloy in at the border of Ferritic—Duplex region onSchaeffler Diagram. At this point the alloy is almost entirely ferritic(e.g., at least 99.5%). Elements like Mo, W and Si are supersaturated inthe ferritic matrix.

The powder of embodiments of the present invention may be produced byconventional powder manufacturing processes. Such processes mayencompass melting of the raw materials followed by water or gasatomization, forming a so called prealloyed powder wherein all elementsare homogeneously distributed within the iron matrix. A major advantagewith a prealloyed powder in contrast to a premixed powder, wherein twoor more powders are mixed together, is that segregation is avoided. Suchsegregation may cause variation in mechanical properties, corrosionresistance etc.

When used for the production of sintered components, the powder ofembodiments of the present invention may be compacted in a conventionaluniaxial compaction equipment at a compaction pressure up to 900 MPa.

Suitable particle size distribution of the stainless steel powder to beused at conventional uniaxial compaction is such that the particle sizeof the powder is between 53 microns and 18 microns such that at least 80wt % of the particles are less than 53 microns and at most 20 wt % ofthe particles are less than 18 microns. Before compaction, the powder ofembodiments of the present invention may be mixed with conventionallubricants, such as, but not limited to, Acrawax, Lithium Stearate,Intralube at a content up to 1 wt %. Other additives mixed in, up to 0.5wt %, may be machinability enhancing agents such as CaF₂, muscovite,bentonite or MnS.

Other methods of consolidation techniques may be utilized such as MetalInjection Molding (MIM), Hot Isostatic Pressing (HIP), extrusion orAdditive Manufacturing techniques such as Binder Jetting, Laser PowderBed Fusion (L-PBF), Direct Metal Laser Sintering (DMLS) or Direct MetalDeposition (DMD)

In a MIM process, suitable particle size distribution of the stainlesssteel powder to be used is such that the particle size of the powder isbetween 26 microns and 5 microns such that at least 80 wt % of theparticles are less than 26 microns and at most 20 wt % of the particlesare less than 5 microns.

In a HIP or extrusion process suitable particle size distribution of thestainless steel powder to be used is such that the particle size of thepowder is between 150 microns and 26 microns such that at least 80 wt %of the particles are less than 150 microns and at most 20 wt % of theparticles are less than 26 microns.

The particle size distribution may be measured by a conventional sievingoperation according to ISO 4497:1983 or by laser diffraction (Sympatec)according to ISO 13320:1999.

After compaction or consolidation, the compacted or consolidated body issubjected to a sintering process at sufficiently high temperatures inthe range of 1150° C. to 1450° C., preferably at sufficiently hightemperatures in the range of 1275° C. to 1400° C. for a period of timeof 5 minutes to 120 minutes. Depending of shape and size of parts to besintered, other period of sintering time such as 10 minutes to 90minutes or 15 minutes to 60 minutes may be applied. The sinteringatmosphere may be vacuum, inert or reducing such as a hydrogenatmosphere, an atmosphere of a mixture of hydrogen and nitrogen ordissociated ammonia. During the sintering process, the supersaturatedelements in ferrite matrix precipitate out as an austenitic phase.Austenite will start precipitating out at the grain boundaries, willgrow with further sintering and will precipitate within the grainitself.

In contrast to other known duplex stainless steel materials, thecomposition of embodiments of the present invention should not formsigma phases or other hard and deleterious phases, e.g., Chi phase andnitrides, during cooling from an elevated temperature, irrespective ofthe cooling rate. For example, the amount of sigma phase or other hardand deleterious phases is less than 0.5%. Forced cooling or quenching isthus not necessary to apply. In this context forced cooling means thatthe sintered parts are subjected to a cooling gas at a pressure aboveatmospheric pressure. Quenching means that the sintered parts aresubmerged into a liquid cooling media.

A microstructure as shown in FIG. 1 will typically be formed containingferrite and austenite. Presence of both phases is responsible forelevated mechanical and corrosion properties. No, or significantlylimited amounts of, deleterious phases such as sigma and chi are formedduring cooling which are normal for current known duplex stainlesssteels. As another consequence, this property will reduce or eliminatethe formation of such phases during welding where the heat affected zone(HAZ) experience varying cooling rates. In another consequence, thiscomposition will limit the precipitation of such phases during processessuch as casting, extrusion, MIM, HIP and additive manufacturing.Embodiments of the invented alloy has shown mechanical and corrosionproperties that are comparable to or exceeding the wrought and PMproducts manufactured with known duplex stainless steel alloys.

In summary, certain advantages of embodiments of this invention mayinclude fewer tendencies to precipitate deleterious sigma and chi phasesthat affect the mechanical and corrosion properties. This isparticularly of interest in welding. Most of the duplex stainless steelcomponents are welded after they are formed. Welding imparts differentcooling rates in different parts of HAZ. These cooling rates tend toprecipitate sigma and chi phases along with nitrides due to nitrogenpresent in the current known alloys. Absence of these phases mayeliminate the post heat treatments, which normally involve annealing attemperatures above 1200° C. followed by rapid cooling. This will in mostcases becomes difficult when parts are welded to a bigger structure,limiting use of duplex stainless steel.

FIGURE LEGENDS

FIG. 1 shows the microstructure of invented sintered stainless steel,austenite and ferrite phases are present in equal proportions in assintered condition, black spots are porosity.

FIG. 2 discloses a comparison of ultimate tensile strength (UTS) andcorrosion properties of the invented sintered stainless steel comparedto 300 and 400 alloys, (SAE grades).

FIG. 3 shows a comparison of mechanical properties of the inventedsintered stainless steel at different sintering conditions

EXAMPLES Example 1

A stainless steel powder, having a particle size below 325 mesh, i.e. 95wt % of the particles passed 45 μm sieve, was mixed with 0.75 wt % ofAcrawax as a lubricant. The chemical analysis of the stainless steelpowder was 0.01 wt % C, 1.52 wt % Si, 0.2 wt % Mn, 0.013 wt % P, 0.008wt % S, 24.9 wt % Cr, 2.0 wt % Cu, 1.3 wt % Mo, 1.0 wt % W, 0.05 wt % N,balance Fe.

The obtained powder mixture was pressed in a uniaxial press andcompacted into transverse rapture strength (TRS) bars, according to ASTMB528-16 at a compaction pressure of 750 MPa. The pressed TRS bars werethen sintered in 100% hydrogen atmosphere at 1343° C. with ramp rate of7° C./minute for 45 minutes. This was followed by furnace cooling atrate 5° C./minute. The samples were then mounted and polished formicrostructure examination. The polished samples were thenelectro-etched with 33% NaOH at 3V for 15 sec. Electro-etch with NaOHreveals the ferrite phase as tan, austenite as white (unaffected) andsigma phases in dark orange at grain boundaries within ferrite matrix.The microstructure observed is as shown in FIG. 1. The microstructureshows approximately 50/50 mixture of ferrite (tan) and austenite(white). There is no sign of any sigma phase (dark orange) in themicrostructure. The black spots are porosity in the sample.

Example 2

Various stainless steel powders according to embodiments of theinvention, and as comparative samples, were produced by water atomizing.The chemical composition of the stainless steel powders are shown intable 1. Stainless steel melts having various chemical compositions weremelted in an induction furnace, the molten metal was subjected to waterstream to obtain steel powder. The obtained powders was then dried andscreened to −325 mesh. The screened powder was −45 microns i.e. 95 wt %of the powder particles were less than 45 microns. The powders were thenmixed with 0.75 wt % of the lubricant Acrawax.

In order to test the mechanical properties i.e. ultimate tensilestrength (UTS), yield strength (YS) and elongation, TS samples (dogbone) per ASTM B925-15 were pressed with a compaction pressure of 750MPa. The bars were then sintered as mentioned in Example 1. The sinteredbars were then tested for mechanical properties per ASTM E8/E8M-16a.Metallographic examination was also conducted in order to establish theratio between austenite and ferrite in sintered samples. The testresults are shown in table 2 in comparison with published data fromsamples of known duplex stainless steels in wrought, (DSS 329 Wrought),and gas atomized and hipped conditions (DSS 329 PM GA).

Table 2 shows that the stainless steel powders according to the presentinvention can be used for producing sintered duplex stainless steelhaving desired mechanical properties.

TABLE 1 chemical compositions of various stainless steel powders, thereproduction method and type of process for producing sintered samples.Chemical analysis [% by weight] Sample Type C Si Mn S P Cr Ni Mo W Cu ON Other Comparative DSS 329 Wrought 0.08 1.00 23-28 2.5-5 1-2 0.08Wrought steel Comparative DSS 329 Water 0.20 0.75 1.00 23-28 2.5-5 1-20.05 0.08 PM WA atomized powder, HIP Comparative DSS Gas 0.03 1.00 2.000.020 0.030 22.0-23.0  4.5-6.5 3.0-3.5 0.75 0.14-0.20 2205 atomized PMGA powder HIP Premix XSS DP1 Water 0.03 2.00 0.10 0.006 0.008 25 5.5 1.31 2 0.2 0.06 PM WA atomized Premix powders⁴, compacted and sinteredInvention XSS DP1 Water 0.01 1.52 0.20 0.013 0.008 24.9 5.5 1.3 1 2 0.150.05 PM WA atomized Prealloy powder, prealloyed compacted and sinteredInvention XSS Water 0.03 1.97 0.10 0.007 0.012 23.4 5.1 1.2 0.9 1.9 0.130.05 DP1-1 atomized powder, prealloyed compacted and sintered InventionXSS Water 0.03 2.12 0.20 0.007 0.012 26.1 5.2 1.3 0.9 3 0.15 0.02 DP1-2atomized powder, prealloyed compacted and sintered Invention XSS Water0.03 1.94 0.20 0.008 0.013 25.1 5.6 1.2 0.8 2 0.15 0.02 0.58 Nb DP1-3atomized powder, prealloyed compacted and sintered Comparative XSS Water0.03 2.14 0.20 0.009 0.015 22.3 5.2 1.3 0.9 1.9 0.16 0.06 0.6 Sn DP1-4atomized powder, prealloyed compacted and sintered ⁴Premix of 316L, 434Land elemental powders of Si, W and Cu.

TABLE 2 mechanical properties and metallographic structure for sinteredsamples produced from stainless steel powders according to table 1.Mechanical Properties Sintering time TS YS TRS Elongation % austenite inSample Type [minutes] [Mpa] [Mpa] [Mpa] [%] ferrite matrix ComparativeDSS 329 Wrought steel Annealed 725 550 25 ~50 Wrought Comparative DSS329 PM WA Water atomized powder, HIP 45 523 460 180 7 0 Comparative DSS2205 PM GA Gas atomized powder, HIP 45 578 427 200 11 ~50 ComparativeXSS DP1 PM WA Water atomized powders⁵, compacted 45 720 700 220 2.5 ~35Premix and sintered Invention XSS DP1 PM WA Water atomized powder,prealloyed 45 776 617 278 8.6 ~60 Prealloy compacted and sinteredInvention XSS DP1-1 Water atomized powder, prealloyed 45 727 504 27511.0 ~50 compacted and sintered Invention XSS DP1-2 Water atomizedpowder, prealloyed 45 809 745 265 2.5 ~50 compacted and sinteredInvention XSS DP1-3 Water atomized powder, prealloyed 45 843 691 257 6.5~45 compacted and sintered Comparative XSS DP1-4 Water atomized powder,prealloyed 45 749 743 218 0.5 ~10 compacted and sintered ⁵Premix of316L, 434L and elemental powders of Si, W and Cu.

An embodiment of the invented powder with composition as in Example 1was also sintered at various temperatures and atmospheres below, to showthe effect on mechanical properties. Such data is plotted in FIG. 3.

-   -   A. 2500° F. for 45 minutes in hydrogen gas    -   B. 2450° F. for 45 minutes in hydrogen gas    -   C. 2450° F. for 60 minutes in hydrogen gas    -   D. 2300° F. for 60 minutes in hydrogen gas    -   E. 2250° F. for 60 minutes in hydrogen gas    -   F. 2250° F. for 60 minutes in dissociated ammonia

Example 3

In order to perform corrosion test, TRS bars as in Example 1 wereproduced along with bars for 316L and 434L as representatives fromaustenitic and ferritic grades. The samples were then tested forcorrosion in 5% NaCl solution at room temperature per ASTM B895-16. Thecorrosion was compared by the hours takes for onset of corrosion on thesamples. The comparative data is plotted in FIG. 2 along with the UTSand YS for these samples. The diameter of the bubbles in the FIG. 3represents the number of hours taken for the start of the corrosion onthe samples. The corrosion test for the invented powder was discontinuedafter 3700 hours as there was no sign of corrosion and it alreadyexceeded 3 times that of 316L samples.

1. A stainless steel powder comprising: up to 0.1% of C, 0.5-3% of Si, up to 0.5% of Mn, 20-27% of Cr, 3-8% of Ni, 1-6% of Mo, up to 3% of W, up to 0.1% N, up to 4% of Cu, up to 0.04% of P, up to 0.04% of S, unavoidable impurities up to 0.8%, optionally one or more of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, rest Fe.
 2. A stainless steel powder according to claim 1 comprising: up to 0.06% of C, 1-3% of Si, up to 0.3% of Mn, 23-27% of Cr, 4-7% of Ni, 1-3% of Mo, 0.8-1.5% of W, up to 0.07% N, 1-3% of Cu, up to 0.03% of P, up to 0.03% of S, unavoidable impurities up to 0.8%, optionally one or more of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, rest Fe.
 3. A stainless steel powder according to claim 1 comprising: up to 0.03% of C, 1.5-2.5% of Si, up to 0.3% of Mn, 24-26% of Cr, 5-7% of Ni, 1-1.5% of Mo, 1-1.5% of W, up to 0.06% N, 1-3% of Cu, up to 0.02% of P, up to 0.015% of S, unavoidable impurities up to 0.8%, optionally one or more of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, rest Fe.
 4. A stainless steel powder according to claim 1 wherein the stainless steel powder is ferritic.
 5. A stainless steel powder according to claim 1 wherein the stainless steel powder is produced by water atomization.
 6. A stainless steel powder according to claim 1 wherein the stainless steel powder is produced by gas atomization.
 7. A stainless steel powder according to claim 1 wherein the particle size of the powder is between 53 microns and 18 microns such that at least 80% of the particles are less than 53 microns and at most 20% of the particles are less than 18 microns.
 8. A stainless steel powder according to claim 1 wherein the particle size of the powder is between 26 microns and 5 microns such that at least 80% of the particles are less than 26 microns and at most 20% of the particles are less than 5 microns.
 9. A stainless steel powder according to claim 1 wherein the particle size of the powder is between 150 microns and 26 microns such that at least 80% of the particles are less than 150 microns and at most 20% of the particles are less than 26 microns.
 10. A stainless steel powder according to claim 1 wherein the powder is a prealloyed powder.
 11. A method for producing a stainless steel powder by water atomization comprising the steps of: providing a molten metal of having a chemical composition corresponding to the chemical composition of the stainless steel powder according to claim 1, subjecting a stream of the molten metal to water atomization, recovery of the obtained stainless steel powder.
 12. A sintered duplex stainless steel having a chemical composition according to claim 1 and wherein the microstructure of the sintered duplex stainless steel is characterized by austenite phase precipitated in ferrite phase.
 13. A sintered duplex stainless steel according to claim 12 wherein the Ni equivalent (Ni_(eq)) is such that 5<Ni_(eq)<11 and the Cr equivalent (Cr_(eq)) is such that 27<Cr_(eq)<38 and wherein Cr_(eq) and Ni_(eq) are calculated according to the formulas: Cr_(eq)=Cr+2Si+1.5Mo+0.75W Ni_(eq)=Ni+0.5Mn+0.3Cu+25N+30C and, wherein Cr, Ni, etc. are the level of each element in the alloy in weight %.
 14. A sintered duplex stainless steel according to claim 12 wherein the pitting resistance equivalent number (PREN) is 28<PREN<33 and wherein PREN is calculated according to the formula: PREN=Cr+3.3Mo+16N and, wherein Cr, Mo and N are the level of each element in the alloy in weight %.
 15. A sintered duplex stainless steel according to claim 12 wherein the microstructure of the sintered duplex stainless steel contains 30-70% austenite.
 16. A sintered duplex stainless steel according to claim 12 wherein the microstructure is characterized by being free from sigma phases and nitrides.
 17. A method for producing a duplex sintered stainless steel comprising the steps of: providing a stainless steel powder according to according to claim 1, optionally mixing the stainless steel powder with a lubricant and optionally other additives, subjecting the stainless steel powder or the mixture to a consolidation process forming a green component, subjecting the compacted green component to a sintering step in an inert or reducing atmosphere or in vacuum at a temperature between 1150° C. to 1450° C., preferably at a temperature between 1275° C. to 1400° C. for a period of time of 5 minutes to 120 minutes, subjecting the sintered component to a cooling step down to ambient temperature.
 18. A method for producing a duplex sintered stainless according to claim 17 wherein the consolidation process includes: uniaxial compaction at a compaction pressure of up to 900 MPa in a die to form a green component, ejecting the obtained compacted green component from the die. 